Method for detecting and monitoring wafer probing process instability

ABSTRACT

A system and method for detecting and monitoring wafer probe stability including the steps of, probing each die on a wafer, and for each die determining whether the result of the probe is a pass or a fail. If the result of a probe is a fail, re-probing the die and determining whether the re-probe is a pass or a fail. Once all the dies have been probed determining the rate of die re-probes that lead to passes, comparing the rate of passes on re-probes to a pre-determined limit, and if the rate of passes on re-probes is greater than the predetermined limit, assigning the probe status as unstable.

FIELD OF INVENTION

The invention relates to a method for detecting and monitoring waferprobing process instability through use of immediate re-probing andimmediate re-probe data.

BACKGROUND

In wafer manufacture probing is a process for testing if dies formed ona wafer are functional. Probing dies on a wafer involves penetrating thesurface of the wafer at predetermined points to make contact with thepads of each die. The probe uses probe needles to make contact with thepads of each die. Once the probe has penetrated the surface of the diean electronic signal is passed through the die from at least one probeneedle to at least one other probe needle. Information received from theprobe needle(s) is used to determine the functionality of the die.

Problems may arise in the probing process due to a number of differentreasons. These reasons include dirt being trapped on the probingneedles, surface residue building up on the probing needles, and theprobing needles becoming bent out of shape.

At present an operator reviews the probe results and judges whetherthere is a probing issue by visual inspection of failed wafer results orlow yield of certain results. For example the operator may suspect thatthere is a probing issue as the number of failed dies per wafergradually increases. The disadvantage of the present method is thatprobing problems usually begin long before they are detected by theoperator. This leads to the probing failure of potentially large numbersof dies that are good dies.

SUMMARY OF INVENTION

It is the object of the invention to provide an improved method forassessing probe stability or to at least provide the public with auseful choice.

In broad terms in one aspect the invention comprises a method fordetecting and monitoring wafer probe stability including the steps of:

-   -   Probing each die on a wafer;    -   For each die determining whether the result of the probe is a        pass or a fail;    -   if the result of a probe is a fail, re-probing the die and        determining whether the re-probe is a pass or a fail;    -   once all dies have been probed determining the rate of die        re-probes that lead to passes;    -   comparing the rate of passes on re-probes to a pre-determined        limit; and    -   if the rate of passes on re-probes is greater than the        predetermined limit, assigning the probe status as unstable.

Preferably the step of assigning the probe status as unstable includessetting a flag on the monitoring device. The step of assigning the probestatus as unstable may further include sounding an alarm and/orproviding an indicator on a monitor. The step of assigning the probestatus as unstable may further include disabling the probe equipment.

Preferably the step of re-probing any die that fails on the first probeis performed a predetermined number of times. In one embodiment there-probing is performed only once for each die that fails on the firstprobe. In alternative embodiments the step of re-probing may beperformed more than once.

Preferably the method further includes creating a probe reference filefor each wafer.

Preferably the probe reference file contains a re-probe limit, re-proberecovery rate information, a bin re-probe limit, a sensitivity limit andthe recovery rate for re-probing.

Preferably the re-probe recovery rate information includes a limitvalue. In one embodiment for wafers with more than a few hundred diesthe limit may be 2%. In other embodiments a limit may be set as threetimes the standard deviation of the re-probe recovery rate frompreviously supplied data.

Preferably the sensitivity limit includes data on the number ofsensitive dies expected in a wafer.

Preferably the recovery rate for re-probing is determined as:$\frac{\begin{matrix}( {{{number}\quad{of}\quad{recovery}\quad{from}\quad{fail}\quad{to}\quad{good}} -}  \\{{recovery}\quad{from}\quad{sensitivity}\quad{limit}\quad{to}\quad{good}}\end{matrix}}{( {{total}\quad{number}\quad{of}\quad{tested}\quad{good}\quad{die}} )}$

Preferably the method further includes the step of generating a reportfrom the probe reference file for each completed wafer test.

Preferably the report includes device identification information andfail to good probe information.

In broad terms in another aspect the invention comprises a system fordetecting and monitoring wafer probe stability including the systemarranged to:

-   -   probe each die on a wafer;    -   for each die determine whether the result of the probe is a pass        or a fail;    -   if the result of a probe is a fail, re-probe the die and        determine whether the re-probe is a pass or a fail;    -   once all the dies have been probed determine the rate of die        re-probes that lead to passes;    -   compare the rate of passes on re-probes to a pre-determined        limit; and    -   if the rate of passes on re-probes is greater than the        predetermined limit, assign the probe status as unstable.

BRIEF DESCRIPTION OF DRAWINGS

The invention including a preferred form thereof is described withreference to the accompanying drawings and without intending to belimiting, wherein;

FIG. 1 shows communication between a tester module and a probe thatprobes the wafer;

FIG. 2 is a flow chart showing the production process flow;

FIG. 3 shows indicative re-probe recovery rates for a particular wafer;

FIG. 4 shows the performance of a probe over a number of wafers; and

FIG. 5 shows an example of a probing report.

DETAILED DESCRIPTION

FIG. 1 shows the communication between the tester module and the probe.The tester module is arranged to control the probe and the probingprocess. Initially the probe tests the first die on the wafer. Theresults are passed to the tester module as shown at the top of FIG. 1.The tester module evaluates the results from the probe.

If the result is a pass then the tester module instructs the probe totest the next die in the wafer. The tester module may also record thatthe die passed the probe.

If the result if a fail the tester moves into the else part of the IFstatement shown in FIG. 1 and instructs the probe to immediatelyre-probe the die. The probe restarts the probe and sends the results tothe tester module as shown at the bottom of FIG. 1. If the result is apass then the tester module instructs the probe to move onto the nextdie in the wafer. If the result is a fail the tester module may eitherbe set up to instruct the probe to again re-probe the die or the testermodule may be set up to instruct the probe to probe the next die. Ineither case the tester module records the result of each test on thedie.

As will be described further below with reference to FIG. 2 the testermodule is set up to instruct the probe to perform a predetermined numberof re-probes on any die that fails the initial probe.

As described above when a die fails the initial probe a re-probe isimmediately undertaken on the die. This has the advantage that the diewill be re-probed when it is still warm, which provices a better testingenvironment for the die. The immediate re-probing has a very low probeindexing time, which saves time in the re-probe as opposed to re-probingall dies that fail the initial probe after the whole wafer has beenprobed. Another advantage of the immediate re-probe is that on there-probe as the probe holes have already been formed the probe canpenetrate further into the wafer and establish better contact with thepads of the die.

FIG. 2 is a flow chart showing the production process for probing awafer. Each wafer or type of wafer has a probing reference file. Thereference file may contain information about the wafer such as the typeof wafer, the number of dies in the wafer, the spacing of the dies inthe wafer, the needle configuration required to test each die in thewafer and the probe results expected in a good wafer. The probereference file may also contain information on the allowable number ofre-probes if the initial probe of a die produces a fail result, there-probe recovery rate limit, and the expected number of sensitive binsin the wafer.

Once the reference file has been read by the tester module, the nextwafer is loaded into the probe device and is probed by the probe asdescribed above with reference to FIG. 1. In the next step, during theprobing, the tester module collects the re-probe data. Once the probehas probed all dies in the wafer the re-probe recovery rate iscalculated for the wafer.

In the final step shown in FIG. 2 the re-probe recovery rate is assessedto determine whether it exceeds the re-probe recovery rate limit asprovided in the probing reference file. If the re-probe recovery rate islower than the re-probe recovery rate limit the probing on the wafer isconsidered complete and the new wafer may be loaded into the probemodule so that the testing process may continue.

If the re-probe recovery rate exceeds the re-probe recovery rate limitan alarm is set and an operator is notified. The operator can theninvestigate the cause of the high re-probe recovery rate and fix anyproblems found. Potential problems with the probe include the probe markoverdrive setting not being optimized, probe mark drift, a bad or dirtyprobe card.

The probing reference file may reside in a central server. The probingreference file contains various predetermined limits for the probingprocess. These limits include the re-probe limit, the re-probe recoveryrate limit, the bin re-probe limit and the sensitive bin.

The re-probe limit is a global setting for the allowed number ofre-probes before the probe is instructed to continue probing the nextdie. For example, if the re-probe limit is set to 1, then only onere-probe will be done for any die that fails the initial probe.Generally setting the re-probe limit to 1 is sufficient.

The re-probe recovery rate limit can be set from experimental resultsand can be set as a percentage. In a production test the re-proberecovery rate can be determined by the following formula:$\frac{( {{number}\quad{of}\quad{recovery}\quad{from}\quad{fail}\quad{to}\quad{good}} )}{( {{total}\quad{number}\quad{of}\quad{tested}\quad{good}\quad{die}} )}$This number can be expressed as a percentage. Usually data generated byproduction tests are skewed as shown in FIG. 3. This Figure shows thatthe re-probe recovery rate is generally small. A re-probe recovery ratelimit can be determined from this data.

For many wafers with more than a few hundred dies a re-probe recoveryrate limit of about 2% is often sufficient. For a wafer with fewer diesor for a wafer that is still undergoing improvement in probing tests(i.e. a sufficient re-probe recovery rate limit has not yet beendetermined) a statistical process control approach of three times thestandard deviation of the collected re-probe rate data may be used.

The re-probe recovery rate limit provides a limit to the percentage oramount of re-probes that change the test status from an initial fail toa pass before an alarm is issued. If the limit is set too low anoperator may be warned that the probe needs to be checked often and whenthere is nothing wrong with the probe. If the limit is set too high theoperator may not be warned often enough and serious probe problems maygo unnoticed leading to unnecessarily failed die.

The bin re-probe limit is the same as the re-probe limit but is specificfor the type of wafer under test. If this limit is set it over-rides there-probe limit. If this limit is not set the re-probe limit is used todetermine the number of re-probes that will be performed on a die thatfails the initial probe.

Some wafers may produce fail results on an initial probe test that areunrelated to any probe conditions and are related to the wafer undertest. These initial fail results need to be discounted from the testresults and the re-probe results as they are unrelated to probeconditions. If a wafer is known to have dies that are sensitive toprobing and may initially fail this is taken into account in thesensitive bin variable. In this way these results can be discounted fromthe number of re-probe passes to reflect the actual rate of recovery ofdies due to any probing issue.

The final formula to determine the re-probe recovery rate is:$\frac{\begin{matrix}( {{{number}\quad{of}\quad{recovery}{\quad\quad}{from}\quad{fail}\quad{to}\quad{good}} -}  \\ {{recovery}\quad{from}\quad{sensitive}\quad{bin}\quad{to}\quad{good}} )\end{matrix}}{( {{total}\quad{number}\quad{of}\quad{tested}\quad{good}\quad{die}} )}$If this rate exceeds the re-probe recovery rate limit an alarm will beactivated and an operator notified. If the rate is lower than there-probe recovery rate limit the next wafer will proceed to the testingprocess.

FIG. 4 shows an example of probe card performance for a number of testedwafers. The wafers under test are shown along the horizontal axis of thegraph and the probe performance as a function of the percentage re-probeis shown on the vertical axis. As can be seen the probe card performanceis generally good and has generally produced a re-probe percentage lessthan the nominal 2% suggested above. However it can be seen that for anumber of cards, each tested one after the other, the re-probe rate ishigher than expected. This suggests a problem with the probe whentesting these cards.

In general the tester will generate a report upon the completion of eachwafer test. In one embodiment the report shows some basic informationabout the tested wafer and also information on other probing projects.The report may also be arranged to show information on other probingprojects. A sample report is shown in FIG. 5. The information in thebottom left hand corner is probing information. The bottom line of thisreport shows the re-probe recovery percentage for the wafer under test.In this example the re-probe recovery percentage is 0.34%, which isgenerally an acceptable level.

The foregoing describes the invention including a preferred formthereof. Alterations and modifications are intended to be incorporatedwithin the scope hereof as defined by the accompanying claims.

1. A method for detecting and monitoring wafer probe stability includingthe steps of: probing each die on a wafer; for each die determiningwhether the result of the probe is a pass or a fail; if the result of aprobe is a fail, re-probing the die and determining whether the re-probeis a pass or a fail; once all the dies have been probed determining therate of die re-probes that lead to passes; comparing the rate of passeson re-probes to a pre-determined limit; and if the rate of passes onre-probes is greater than the predetermined limit, assigning the probestatus as unstable.
 2. A method for detecting and monitoring wafer probestability as claimed in claim 1 wherein the step of assigning the probestatus as unstable includes setting a flag on the monitoring device. 3.A method for detecting and monitoring wafer probe stability as claimedin claim 2 wherein the step of assigning the probe status to unstablefurther includes sounding an alarm and/or providing an indicator on amonitor.
 4. A method for detecting and monitoring wafer probe stabilityas claimed in claim 2 wherein the step of assigning the probe status tounstable further includes disabling the probe equipment.
 5. A method fordetecting and monitoring wafer probe stability as claimed in claim 1wherein the step of re-probing any die that fails on the first probe ispreformed a predetermined number of times.
 6. A method for detecting andmonitoring wafer probe stability as claimed in claim 5 whereinre-probing is preformed only once for each die that fails on the firstprobe.
 7. A method for detecting and monitoring wafer probe stability asclaimed in claim 5 wherein the step of re-probing may be performed morethan once.
 8. A method for detecting and monitoring wafer probestability as claimed in claim 1 further including the step of creating aprobe reference file for each wafer.
 9. A method for detecting andmonitoring wafer probe stability as claimed in claim 8 wherein the probereference file contains a re-probe limit, re-probe recovery rateinformation, a bin re-probe limit, a sensitivity limit and the recoveryrate for re-probing.
 10. A method for detecting and monitoring waferprobe stability as claimed in claim 9 wherein the re-probe recovery rateinformation includes a limit value.
 11. A method for detecting andmonitoring wafer probe stability as claimed in claim 10 wherein forwafers with more than a few hundred dice the limit is 2%.
 12. A methodfor detecting and monitoring wafer probe stability as claimed in claim10 wherein the re-probe rate recovery limit is set as three times thestandard deviation of the re-probe recovery rate from previouslysupplied data.
 13. A method for detecting and monitoring wafer probestability as claimed in claim 9 wherein the sensitivity limit includesdata on the number of sensitive dies expected in a wafer.
 14. A methodfor detecting and monitoring wafer probe stability as claimed in claim 9wherein the recovery rate for re-probing is determined as:$\frac{\begin{matrix}( {{{number}\quad{of}\quad{recover}\quad{from}\quad{fail}\quad{to}\quad{good}} -}  \\ {{recovery}\quad{from}\quad{sensitivity}\quad{limit}\quad{to}\quad{good}} )\end{matrix}}{( {{total}\quad{number}\quad{of}\quad{tested}\quad{good}\quad{die}} )}$15. A method for detecting and monitoring wafer probe stability asclaimed in claim 8 wherein the method further includes the step ofgenerating a report from the probe reference file for each completedwafer test.
 16. A method for detecting and monitoring wafer probestability as claimed in claim 15 wherein the report includes deviceidentification information and fail to good probe information.
 17. Asystem for detecting and monitoring wafer probe stability including thesystem arranged to: probe each die on a wafer; for each die determinewhether the result of the probe is a pass or a fail; if the result of aprobe is a fail, re-probe the die and determine whether the re-probe isa pass or a fail; once all the dies have been probed determine the rateof die re-probes that lead to passes; compare the rate of passes onre-probes to a pre-determined limit; and if the rate of passes onre-probes is greater than the predetermined limit, assign the probestatus as unstable.
 18. A method for detecting and monitoring waferprobe stability as claimed in claim 3 wherein the step of assigning theprobe status to unstable further includes disabling the probe equipment.